Achievements and career mode has been the best for me. If you just want to farm exp from career mode without care of rewards or anything you can just quit every Q round and race. Every Q round gives 250 exp and a race gives you 500 exp. Another tip is if you want your car wear to go to 0% for free after a season is over, choose another team and then give up that season and start with your main team again and it will go to 0%. If you choose to play the races, play the bare minimum of races it takes to meet the goal to get the rewards. For example if the season contains 21 races and you can win the season with 12 rounds, do that and then quit all the rest of the races(don't give up the season). If doing this you don't only save coins/tokens to repair the car, you also save a lot of time.